Our research on V(D)J recombination, the rearrangement of gene segments that is used to assemble intact immunoglobulin and T cell receptor coding regions, has continued in the past year. This process is initiated by a bipartite recombinase made up of the RAG1 and RAG2 proteins, which act together to introduce double-stranded breaks into the DNA adjacent to the coding segments. In the same raction they convert the ends of the coding DNA into covalently joined DNA hairpins. Similar hairpins are formed by some related transposases, such as the large group of eukaryotic hAT transposases. and we are following up this analogy to try to explain the large distortion of DNA structure that is necessary to make hairpinning possible. We have identified two tryptophan residues that may be involved in binding a base extruded from the DNA, thus making the DNA more flexible. In support of this model, we have found that removal of a critical base at the cleavage site ( to generate an abasic structure) allows cutting of the DNA under conditions where it would not normally occur, and can also rescue tryptophan mutations that otherwise block cleavage. [unreadable] Work is also continuing on constructing variants of the RAG1 and RAG2 proteins, to be tested for their ability to crystallize in a specific complex with DNA.